Electronics Club

Verilog Lecture Series IIT KANPUR

Pre-requisite

• Basics of any programming language

• Easy for those with some programming experience

• Some background in digital logic will also be helpful

Need

• Execute different code blocks simultaneously as opposed to sequential execution of most programming languages.

• Make circuits easier to verify and reduces error

Let’s Get Started

Terminology

• Verilog: Hardware Description Language (HDL)

• Used to model electronic systems

• Modules :- “ Black Boxes ”

• Ports :- input , output , inout

• Case Sensitive

• Synthesis

Synthesis

Drivers

• A driver is a data type which can drive a load.

• Basically, in a physical circuit, a driver would be anything that electrons can move through

• Two types of Drivers

• The first type of driver – one that can store a value example flipflop.

• It is called a “reg” in Verilog (short for "register")

• The second data type is one can not store value, but connects two points e.g. wire.

• It is called a “wire”

Modules

Look at the counter block in the picture

• we can see that it has got a name (“4029")

• Also input/output ports (4 bit output and 1 bit clock)

Defining Modules

• module <specify type>(< port list >)

• module 4029 (clk,out,reset,enable); or

• module 4029 (clk,a,b,c,d,reset,enable);

Input and ouput

input clk; input clk;

input reset; or input reset;

input enable; input enable;

output a,b,c,d; output [3:0] out;

Driver type

wire clk; wire clk;

wire reset; or wire reset;

wire enable; wire enable;

reg a,b,c,d; reg [3:0] out;

Summary

• We learnt how a block/module is defined in Verilog.

• We learnt how to define ports and port directions.

• We learnt how to declare vector/scalar ports.

Operators

• Similar to C

Operator Type Operator Symbol Operation Performed

Arithmetic * Multiply

/ Division

+ Add

- Subtract

% Modulus

+ Unary plus

- Unary minus

Logical ! Logical negation

&& Logical and

|| Logical or

Relational > Greater than

< Less than

>= Greater than or equal

<= Less than or equal

Equality == Equality

!= inequality

Reduction ~ Bitwise negation

~& nand

| or

~| nor

^ xor

^~ xnor

~^ xnor

• Note that the ++ and -- operators are not supported in Verilog

Shift >> Right shift

<< Left shift

Concatenation { } Concatenation

Conditional ? conditional

Control Statements

• if, else, repeat, while, for, case

• looks exactly like C

• Difference : » { begin

» } end

» endcase

• begin and end act like curly braces in C/C++

If else

if (enable == 1'b1) begin data = 10; //Decimal assigned address = 16'hDEAD; // Hexadecimal end else begin enable = 1'b1; // Binary data = 32'b0; end

case(address) 0 : $display (“hi ”); 1 : $display (“i am smart "); 2 : $display (“electronics club"); default : $display (“its easy"); endcase

While

while (free_time) begin $display ("I have lots of free time"); end

What’s New Then ???

• Always Block

• Initial Block

• Assign

• Blocking vs Non-Blocking

Always

• Always block

• All blocks marked always will run simultaneously

• You can have two or more always blocks in a program going at the same time

• The @ symbol after reserved word ' always', indicates that the block will be triggered "at" the condition in parenthesis after symbol @

Note: can not drive wire data type, but can drive reg and integer data types

always @ (a or b or sel) // 2 * 1 MUX begin y <= 0; if (sel == 0) begin y <= a; end else begin y <= b; end end

Initial Block

• An initial block is executed only once when simulation starts

• This is useful in writing test benches

• If we have multiple initial blocks, then all of them are executed at the beginning of simulation

initial begin clk <= 0; reset <= 0; req_0 <= 0; req_1 <= 0; end

Summary

• While, if-else, case(switch) statements are the same as in C language

• If-else and case statements require all the cases to be covered for combinational logic

• For-loop is the same as in C, but no ++ and -- operators

Program

• Gates

• Counter

My_gate1

module my_and1 (a,b,out)

input a;

input b;

output out;

wire a;

wire b;

wire out;

My_and2

module my_and2 (a,b,out)

input [4:0] a;

input [4:0] b;

output [4:0] out;

wire [4:0] a;

wire [4:0] b;

wire [4:0] out;

Counter

module my_counter1 (clk, reset,enable,out); input clk; input reset; input enable; ouput [4:0] out; wire clk; wire reset; wire enable; reg [4:0] out ;

Whats the difference ?

my_and1 is a bit and gate

my_and2 is a 4 bit and gate

done with the module declaration,port list, driver type